The present invention relates to wireless data communication. More particularly, the present invention relates to a novel and improved method and apparatus for power control in a wireless communication system.
In a wireless communication system, power control is used to ensure that the reverse channel signals from mobile users are centrally received at approximately the same power level. In a Code Division Multiple Access (CDMA) system particularly, power control prevents one user from interfering with those sharing a same Radio Frequency (RF) band. Power control adjusts the transmit power of each mobile unit so that each mobile unit achieves an acceptable Signal-to-Noise Ratio (SNR). In other words, power control seeks to solve the xe2x80x9cnear-farxe2x80x9d problem in spread-spectrum multiple access systems and thus increase capacity of the system.
In practice, the power control seeks to adjust the transmit power in response to the effects of fading. Ideally, the power control adjustments allow the mobile unit to track the fading, and provide sufficient power to achieve a desired SNR and overcome the effects of fading, while at the same time avoiding the use of excess power that interferes with other users.
Traditional methods of power control provide power control adjustments at a fixed step size. Several methods determine the step size based on analysis of power as a function of vehicular speeds, as fading is typically a function of the movement of the mobile unit. A problem exists in the delay between a probe of the transmitted signal and the adjustment of transmit power. The delay is introduced by the time required for the base station to receive and analyze the transmitted signal from the mobile unit, the time to generate a power control signal from the base station, and the time for the mobile unit to make the adjustment indicated by the power control signal. The fixed step size does not accurately track the fading over all speeds.
Therefore, a need exists for a method for power control that accurately tracks the channel fading and provides adjustments that adapt to changing conditions.
The disclosed embodiments provide a novel and improved method for power control in a wireless communication system. In one embodiment, a CDMA wireless communication system implements a method for power control using a variable step size adjustment. The power control signal transmitted from the base station indicates whether the transmit power of the mobile unit is to increase or decrease. Step size refers to the size of the change in transmit power effective in response to the power control signal. At the mobile unit, the process compares each received power control signal to at least one (1) previously received power control signal from the transmitter. If the comparison is TRUE, i.e., the previous power control signal is the same sign as the current power control signal, an increment/decrement value is added to the step size. Otherwise, the step size is returned to a default value. In one embodiment, the method has a maximum step size that determines a ceiling and a minimum step size that determines a floor. In the exemplary embodiment, the default values are determined by the polarity of the direction change. In other words, increases following a decrease are treated differently than decreases following an increase. Additionally, step sizes associated with power increases are incrementally increased. Step sizes associated with power reductions are incrementally decreased. The diversity of approach provides smart tracking that allows the mobile unit to track the fading at the receiver more quickly. The power control signal received from the base station is used to make these step size decisions without requiring any modifications to the base station generated signal.